Resolve the "INTERMediator\FileMakerServer\RESTAPI\Supporting\CommunicationProvider::getOAuthIdentifier() never returns string so it can be removed from the return type" error detected by PHPStan

Resolve the "INTERMediator\FileMakerServer\RESTAPI\Supporting\CommunicationProvider::getOAuthIdentifier() never returns string so it can be removed from the return type" error detected by PHPStan #242